In many applications it is desirable to utilize beams of charged particles which impinge upon materials in the processing thereof. For example, electron beams of relatively high intensity are often used to irradiate medical instruments and materials for sterilization purposes. Further, beams of ions or electrons are often used to impinge upon plastic materials in the processing thereof, as for polymerizing such materials. For example, electron beams having energies typically in the range of hundreds of keV up to as high as 10 MeV, or thereabouts, are used for such sterilization or polymerization irradiation. The beams which are used may often have a momentum spectrum with a width typically of about .+-. 10 percent.
In such systems, for example, the beam of charged particles may be obtained from a linear accelerator which produces a beam having substantially parallel rays of such particles and which is usually mounted so as to produce a horizontally oriented beam. The material which is to be irradiated is generally moving along a horizontal conveyor belt so that the irradiation beam from the accelerator must be deflected from its horizontal orientation to a vertical orientation, while at the same time the beam is moved back and forth, or scanned, in a periodic manner so that it scans the conveyor and, hence, the material placed thereon.
Because the particles contained in the beam have different momenta, they are deflected at different angles as they pass from the horizontal to the vertical direction. Accordingly, when the particles reach the conveyor target, they are dispersed in proportion to their momenta, and it is necessary to compensate in some manner for such momentum dispersion in order to limit the size of the metal-foil window through which the beam is emerging from the vacuum system into the air.
Further, when the beam is scanned and moved periodically back and forth in the scanning plane, additional dispersion of the particles occurs since the higher momentum particles are scanned less (i.e., through a smaller scan angle) than the lower momentum particles, and means must be provided for causing the particles to be scanned over a scanning amplitude which is substantially the same for all momenta.
Moreover, the diameter of the beam (i.e., the beam spot size or spatial focussing thereof) at the target must be appropriately adjusted to provide for suitable overlap of the scanning paths as the conveyor and the target materials move thereunder. Thus, if the spot size is too small, sufficient overlap cannot be obtained, while if it is too large, the zones of diminishing average intensity become too wide along the edges of the conveyor belt at the ends of the scan amplitude. Accordingly means must be provided for appropriately adjusting the beam size at the target plane.
Moreover, the means for providing dispersion and beam size adjustment and compensation must be appropriately compatible so that their interactions produce the desired overall beam deflection, scanning and spot size characteristics, even when the distance between the beam control system and the target at the conveyor varies.
One approach to the problem which has been offered in the prior art is discussed in U.S. Pat. No. 3,193,717 issued to C. S. Nunan on July 6, 1965. In the Nunan system a deflecting magnet has an edge which is contained in a rotatable section for changing the angle of inclination between the particle beam and the magnet to change the trajectories of particles of different energies through the same angle. The deflection magnet is followed by a scanning magnet whose pole faces have a magnetic field gradient therebetween to deflect all the particles of a particle beam with a particle energy gradient through approximately the same angle. A quadrupole magnet is provided ahead of the deflecting magnet to adjust the size of the irradiating spot of particles at the target.
Because Nunan provides for scanning to take place after deflection of the beam a relatively large scanning magnet is required in order to provide a sufficiently wise scan path at the target. Moreover, the scan magnet has a complex configuration since it is also used to correct for dispersion effects of the beam in the scanning plane due to the deflection thereof by the deflecting magnet. Dispersion correction in the deflecting plane is taken care of by adjusting the exit angle of the beam at the output of the deflection magnet. However, since the configuration of the deflection magnet places a physical limit on the size of such angle, complete correction for deflecting plane dispersion at the target cannot be achieved without undesirable difficulty. Because of the inability to completely correct for deflecting plane dispersion the size of the output window through which the particles pass from the deflection/scanning system to the target is larger than would be required if complete correction is made. Because the window must withstand a required amount of pressure it cannot be made too large or else it will be subject to possible breakage during operation.
Hence, a more effective and less costly design to overcome the deficiencies of the Nunan system is desired.